1. Field of the Invention
The present invention relates to a method for estimating a distribution width of Curie temperatures of a plurality of magnetic grains that form a magnetic recording layer of a magnetic recording medium used in thermally-assisted magnetic recording.
2. Description of the Related Art
In the field of magnetic recording using a head and a medium, further improvement is demanded in the performance of a thin film magnetic head and a magnetic recording medium in conjunction with a growth of the high recording density of a magnetic disk device. As the thin film magnetic head, currently, a composite-type thin film magnetic head is widely used having a structure in which a magnetoresistive (MR) element for reading and an electromagnetic transducer element for writing are laminated.
The magnetic recording medium is a discontinuous medium in which magnetic grains that are formed from a magnetic material are aggregated, and each of the magnetic grains forms a single magnetic domain structure. In this magnetic recording medium, one recording bit is composed of a plurality of magnetic grains. Therefore, in order to increase the recording density, the volume of a magnetic grain should be reduced to reduce asperity of a boundary between adjacent recording bits. However, when the magnetic grains are made small, a problem occurs that along with the decrease of the volume of a magnetic grain, thermal stability of the magnetization of the magnetic grain decreases.
As a countermeasure to this problem, it is conceivable to increase magnetic anisotropy energy Ku of a magnetic grain. However, the increase in Ku results in an increase in an anisotropy magnetic field (coercive force) of the magnetic recording medium. On the other hand, an upper limit of a writing magnetic field intensity of a thin film magnetic head is largely determined by a saturation magnetic flux density of a soft magnetic material that composes a magnetic core in the head. Therefore, when the anisotropy magnetic field of the magnetic recording medium exceeds a tolerance value determined from the upper limit of the writing magnetic field intensity, writing becomes impossible. Currently, as a method for solving such a thermal stability problem, a so-called thermally-assisted magnetic recording system has been proposed in which, while a magnetic recording medium formed from a magnetic material with a large Ku is used, the anisotropy magnetic field is reduced to perform writing by heating the magnetic recording medium to a temperature near the Curie temperatures of the magnetic grains immediately before a writing magnetic field is applied.
A magnetic recording medium (magnetic disk) 301 used in the thermally-assisted magnetic recording has a configuration in which, for example, as illustrated in FIG. 1, a soft magnetic under layer 301b, a heat sink layer 301c, an intermediate layer 301d, a magnetic recording layer 301e and a protective layer 301f are laminated in this order on a disk substrate 301a. The magnetic recording layer 301e is fabricated on the intermediate layer 301d by using a film formation method such as sputtering, and is formed as an aggregate of a plurality of magnetic grains that are composed of a magnetic material such as a single metallic material and an alloy material composed of two or more kinds of metals.
In such a magnetic recording medium 301, variations may occur in the size and shape of the plurality of magnetic grains that form the magnetic recording layer 301e due to film formation conditions and the like. Further, in the case where the magnetic grains are composed of an alloy material that is composed of two or more kinds of metals, the metal composition of the alloy material may be non-uniform with respect to each of the magnetic grains. As a result of these, the Curie temperature of each magnetic grain (the temperature at which the magnetization of the magnetic grain disappears) is different and a distribution of a predetermined width occurs in the Curie temperature.
The distribution width of the Curie temperatures of the magnetic grains (the difference between the maximum value (maximum Curie temperature) and the minimum value (minimum Curie temperature) among the Curie temperatures of the plurality of magnetic grains) that may occur in this manner affects characteristics of a thermally-assisted magnetic recording device.
For example, in a case where the thermally-assisted magnetic recording device includes a magnetic head that can heat a magnetic recording medium on an upstream side in a medium traveling direction of the magnetic recording medium and apply a recording magnetic field to the magnetic recording medium on a downstream side in the medium traveling direction of the magnetic recording medium, the magnetization transition point of each of the magnetic grains that form the magnetic recording layer of the magnetic recording medium in magnetic recording using the magnetic head should be considered.
In this case, for magnetic grains having Curie temperatures in a low temperature region (near the minimum Curie temperature), the magnetization transition point is located on a relatively downstream side in the medium traveling direction. On the other hand, for magnetic grains having Curie temperatures in a high temperature region (near the maximum Curie temperature), the magnetization transition point is located on a relatively upstream side in the medium traveling direction. Therefore, according to the distribution width of the Curie temperatures of the magnetic grains, the width of the magnetization transition points of the magnetic grains in the magnetic recording medium changes. Therefore, the larger the distribution width of the Curie temperatures of the plurality of magnetic grains that form the magnetic recording layer of the magnetic recording medium provided in the thermally-assisted magnetic recording device is, the easier the jitter associated with unintended magnetization reversal and the like occurs, and the SN ratio and bit error rate (BER) tend to deteriorate.
To meet the demand for even higher recording density in a thermally-assisted magnetic recording device, it is important not only to improve recording characterizations of the magnetic head but also to reduce recording errors of the magnetic grains that form the magnetic recording layer of the magnetic recording medium. Therefore, with respect to the magnetic recording medium with which the thermally-assisted magnetic recording device is configured, performing pass/fail determination based on the distribution width of the Curie temperatures of the plurality of magnetic grains that form the magnetic recording layer and, for this purpose, reliably obtaining the distribution width of the Curie temperatures of the magnetic grains are important for solving the above problems.
As a method for measuring the Curie temperature of the magnetic recording layer in the magnetic recording medium, in general, a method involving hysteresis loop measurement at a high temperature is known. However, in this method, only an average value of the Curie temperatures of the plurality of magnetic grains that form the magnetic recording layer (the temperature at which magnetization disappears in a majority of the magnetic grains among the plurality of magnetic grains) is obtained. The distribution width of the Curie temperatures that arises due to the difference in the Curie temperature for each magnetic grain cannot be measured.